Wear and corrosion resistant coatings on surface of cast A356 aluminum alloy by plasma electrolytic oxidation in moderately concentrated aluminate electrolytes

Plasma electrolytic oxidation of a cast A356 aluminum alloy was carried out in aluminate electrolytes to develop wear and corrosion resistant coatings. Different concentrations of 2, 16 and 24 g/L NaAlO₂ solutions and a silicate electrolyte (for comparison) were employed for the investigation. Wear performance and corrosion resistance of the coatings were evaluated by WC (tungsten carbide) ball-on-flat dry sliding tests and electrochemical methods, respectively. The results show that the coating formed for a short duration of 480 s in 24 g/L NaAlO₂ solution generated the best protection. The coating sustained 30 N load for sliding time of 1800 s, showing very low wear rate of ～4.5×10^?7 mm³/(N·m). A low corrosion current density of ～8.81×10^?9 A/cm² was also recorded. Despite low α-Al₂O₃ content of the coating, the compact and nearly single layer nature of the coating guaranteed the excellent performances.